As is generally known, in a multi-axle vehicle having three or more tire axles (six tired wheels), it is possible to reduce pressure of ground contact of each tired wheel because the mass of the vehicle can be distributed to the multiple axles, whereby it can be frequently used as a tire type carrier vehicle capable of traveling even on, for example, a soft ground. In this case, a suspension apparatus of a multi-axle vehicle is generally made an independent suspension for each tired wheel for the purpose of equalizing pressure of ground contact of each tired wheel even on the ground with bumps and dips, thereby making it possible to travel on an uneven ground and also making it possible to travel the whole distance on a soft ground without damaging the ground.
In order to simplify the explanation, a tire axle is used for collectively calling a pair of left and right tired wheels and an axle for supporting the left and right tired wheels. Further, an axle is not limited to an integrated type of axle (rigid axle) for connecting the left and right tired wheels, but includes each of left and right independent suspension type of axles for independently supporting left and right tired wheels.
A structure for responding to a ground surface with bumps and dips is also considered in a track-laying vehicle other than a multi-axle vehicle. For example, Japanese Patent No. 3049511 (especially on page 4 and FIG. 1) describes a structure in which a pair of left and right triangular crawler devices are placed at each of a front part and a rear part of a vehicle, but prior art limited to suspension apparatuses of multi-axle vehicles will be explained below. Based on FIG. 15, an example of a suspension apparatus of a multi-axle vehicle according to a prior art will be explained with a tire type carrier vehicle having four tire axles cited as an example. In a carrier vehicle 90, four tire axles 91, 92, 93 and 94 are mounted to a vehicle body 95 constituted of a frame 95a, a rear deck 95b, a driver's cab 95c and the like via suspension apparatuses 91b, 92b, 93b and 94b, respectively. The tire axles 91, 92, 93 and 94 have tired wheels 91a, 92a, 93a and 94a, respectively. Since a right side of the vehicle body is the same as the above description, the explanation thereof will be omitted, and only a left side of the vehicle body will be explained hereinafter.
In the above-described constitution, the mass of the vehicle body 95 is distributed to the four tire axles 91, 92, 93 and 94, and therefore pressure of ground contact of the tired wheels 91a, 92a, 93a and 94a is low. Further, the tire axles 91, 92, 93 and 94 individually have the suspension apparatuses 91b, 92b, 93b and 94b, respectively, and therefore the mass of the vehicle body 95 is also distributed to each of the tire axles 91, 92, 93 and 94 on a ground having bumps and dips. As a result of these, it is made possible for the carrier vehicle 90 to travel on an uneven ground and travel the whole distance on a soft ground without damaging the ground.
However, in the suspension apparatuses 91b, 92b, 93b and 94b, there are several problems occurring due to the multi-axles and independent suspensions. A first problem will be explained initially based on FIG. 16A and FIG. 16B. In FIG. 16A, a phenomenon, in which a front part of the vehicle body 95 is lifted in a direction of the arrow U (hereinafter, called squat), is caused by moment by an inertia force of a mass G of the vehicle 95 and its road clearance H1, when the carrier vehicle 90 starts, as is generally known. For the same reason, a phenomenon, in which the front part of the vehicle body 95 sinks in a direction of the arrow N (hereinafter, called nose down), is caused when the carrier vehicle 90 stops. On that occasion, the squat is suppressed by a supporting force of the rearmost suspension apparatus 94b, and the nose down is suppressed by a supporting force of the suspension apparatus 91b at the forefront.
Since the carrier vehicle 90 has the four tire axles, the mass of the vehicle body 95 is made small by being distributed by being divided into substantially four equal parts of respective axle loads P1, P2, P3 and P4 of the four tire axles 91, 92, 93 and 94, and the supporting force of each of the suspension apparatuses 91b, 92b, 93b and 94b is made small corresponding to this. As a result of them, the squat and nose down of the carrier vehicle 90 tend to be larger than those of the other ordinary two-axle vehicles.
In FIG. 16B, the vehicle body 95, which is long in a longitudinal direction, has a large inertia moment Ip within a vertical surface in the longitudinal direction, and therefore it cannot fully follow the movement of each of the tire axles 91, 92, 93 and 94 when the carrier vehicle 90 travels on an uneven ground, thus causing swing shown by the arrow P (hereinafter, called pitching). Meanwhile, the supporting force of each of the suspension apparatus 91b at the forefront and the suspension apparatus 94b at the rear end to suppress the pitching is set to be small. Further, in the raised ground as shown in the drawing, a phenomenon, in which the suspension apparatuses 91b at the forefront and 94b at the rear end are in the extended state, as a result that the tire axles 92 and 93 are propped up at center portions, and the supporting forces become smaller. As a result of them, the carrier vehicle 90 cannot fully suppress pitching in travel on an uneven ground in some cases, and there arises the problem that the pitching has to be converged by making the traveling speed extremely low in such a case.
A second problem will be explained based on FIG. 17. In FIG. 17, limit height S of a step, which the carrier vehicle 90 can get over, is generally D/2 that is a half of a diameter D of the tired wheel 91a at the forefront. Consequently, in order to enhance mobility on an uneven ground, it is necessary to increase the diameter D of the tired wheel 91a. On the other hand, in the carrier vehicle 90, four of the tire axles are placed for the purpose of enabling the carrier vehicle 90 to travel the while distance on a soft ground in an uneven ground, whereby the axle load per axle is made small. Consequently, there is not an enough space to increase the diameters D of the tired wheels 91a, 92a, 93a and 94a. As a result of them, there is the problem that the carrier vehicle 90 cannot get over a large step.
A third problem will be explained based on FIG. 18A to FIG. 18D. First, in FIG. 18A, when the carrier vehicle 90 gets over a step on a soft ground, the tired wheel 91a is pressed against a tip end S1 of the step with the force P1 and is lifted upward, and the load distribution to the tired wheel 91 becomes large. In FIG. 18B, the axel load of the tire axle 91 is large, and therefore the tired wheel 91a sinks to form a track T1. The tire axle 92 is pressed against a tip end S2 of the step with the force of P2 and lifted upward, and the load distribution to the tire axle 92 becomes large.
In FIG. 18C, the axle load of the tire axle 92 is large, and therefore the tired wheel 92a sinks to deepen the track to T2. The tire axle 93 is pressed against a tip end S3 of the step with the force of P3 and lifted upward, and the load distribution to the tire axle 93 becomes large. In FIG. 18D, the axle load of the tire axle 93 is large, and therefore the tired wheel 93a sinks to deepen the track to T3. This results in the state in which the tip end S1 of the step at an intermediate portion of the left and right tracks contacts a center portion of the tire axle 93 or the tire axle 94 and the carrier vehicle cannot move (hereinafter, called a stack state). Namely, there is the problem that the phenomenon, in which every time each of the four tire axles 91, 92, 93 and 94 gets over the step in succession, the axle load concentrates on each of the axles, is repeated, whereby the deep track T3 is formed, and the carrier vehicle 90 is easily brought into the stack state.